Unconventional Secretion, Unconventional Solutions

Most eukaryotic secretory proteins are directed into the endoplasmic reticulum (ER) by an amino-terminal signal peptide (leader sequence) and progress to the cell surface through vesicular flow via the Golgi apparatus (1, 2). However, many claims have been made for a number of diverse membrane and soluble proteins that lack a typical signal peptide being transported from the cytoplasm or nucleus independently of this classical ER-Golgi route, so-called unconventional secretion (3–5). Some examples, such as secretion of the yeast a-factor mating pheromone by a plasma membrane peptide transporter (6), are well understood. However, other examples are of proteins that are inefficiently secreted, and the concern is that in some experiments this “export” is due to cell lysis rather than actual secretion (7). Here we summarize the key evidence for unconventional secretion, and pose questions that remain to be addressed.